arXiv subjects

Radu Andrei

Publications and source records attributed to Radu Andrei.

5 recordsLinked to original sources

arXiv2026

Universal magnetic energy scale in the doped Fermi-Hubbard model

Magnetic correlations of doped Mott insulators hold the key to the unusual characteristics of many quantum materials. Recent experiments with ultracold atoms in optical lattices have provided new information about the magnetic properties of the Fermi-Hubbard model on a square lattice. We demonstrate that recent measurements indicate that a single doping-dependent energy scale determines both static correlations and dynamical response of these systems. To understand these experimental findings, we employ a self-consistent formalism to describe the coupling between antiferromagnetic magnons and doped holes, and we uncover the emergence of a universal magnetic energy scale at finite doping, which we denote by $J^*$. We present the single- and two-magnon spectral properties at finite doping and discuss the appearance of a bimagnon peak in lattice-modulation spectroscopy, at frequencies set by $J^*$. Furthermore, we argue that this same energy scale sets the onset of pseudogap phenomena, leading to the hypothesis $k_BT^* = c J^*$, with $c$ an order one number. We identify another low-energy scale emerging from our analysis of magnetic excitations, and argue that it controls the stability of N\'{e}el order at the lowest temperatures, ultimately driving a transition to an incommensurate spin-density-wave at finite doping. We discuss the relation between this low-energy scale and the nature of fermionic quasiparticles. Our analysis suggests that stability of the commensurate antiferromagentic phase at finite doping can be controlled experimentally by introducing additional quasiparticle broadening via disorder or low-frequency noise.

cond-mat.str-el↗

arXiv2026

Toward the Thermodynamic Limit: Neural Operators for Non-equilibrium Dynamics of Mott Insulators

Mott insulators exhibit complex photoexcitation dynamics under intense optical driving, with potential implications for carrier multiplication beyond the Shockley-Queisser limit. Probing these nonequilibrium processes requires access to the thermodynamic limit, where the number of lattice sites becomes arbitrarily large, but conventional solvers are constrained to small systems due to the exponential growth of the Hilbert space. Fourier Neural Operators (FNOs), originally developed for solving partial differential equations, naturally accommodate inputs of varying resolution and are capable of capturing nonlocal effects. Here, we employ FNOs to learn the mapping from noise-perturbed ground-state momentum distributions to their post-pulse counterparts across a range of interaction strengths and driving parameters. Trained only on small lattices, the model generalizes zero-shot to much larger systems, producing physically reasonable momentum distributions well beyond the reach of numerical solvers. Specifically, the model can predict momentum distribution for a 1024x1024 system within a few seconds that matches the theoretical behavior of key observables, whereas direct numerical simulations have so far been restricted to edge sizes of ~30. These results demonstrate the potential of neural operators to directly access large-scale nonequilibrium dynamics, providing a new pathway toward the thermodynamic limit in strongly correlated materials.

cond-mat.str-el↗

arXiv2025

Subgap pumping of antiferromagnetic Mott insulators: photoexcitation mechanisms and applications

We study the behavior of the 2D repulsive Hubbard model on a square lattice at half filling, under strong driving with ac electric fields, by employing a time-dependent Gaussian variational approach. Within the same theoretical framework, we analytically obtain the conventional Keldysh crossover between multiphoton and tunneling photoexcitation mechanisms, as well as two new regimes beyond the Keldysh paradigm. We discuss how dynamical renormalization of the Mott-Hubbard gap feeds back into the photoexcitation process, modulating the carrier generation rate in real time. The momentum distribution of quasiparticle excitations immediately after the drive is calculated, and shown to contain valuable information about the generation mechanism. Finally, we discuss experimental probing of the pump-induced nonequilibrium electronic state.

cond-mat.str-el↗

arXiv2022

The spin-one Motzkin chain is gapped for any area weight $t<1$

We consider the spin-one Motzkin chain with area weight $t>0$. We resolve three questions from the literature about this model. We prove (i) existence of a uniform spectral gap for all $t<1$ as conjectured by Zhang--Ahmadein--Klich \cite{zhang2017novel} (ii) an explicit formula for the long-distance limit of the string order parameter, which implies it is non-vanishing at small $t$, confirming a conjecture by Barbiero et al. \cite{barbiero2017haldane}, and (iii) that gaplessness for $t>1$ is robust and extends to hard boundary conditions, answering a question of Zhang--Klich \cite{zhang2017entropy}. These conclusions rest on an effective approximate description of the ground states of finite open Motzkin chains in terms of height-controlled imbalanced Motzkin walks.

quant-ph↗

arXiv2018

Topological Phononic Logic

Topological metamaterials have robust properties engineered from their macroscopic arrangement, rather than their microscopic constituency. They can be designed by starting from Dirac metamaterials with either symmetry-enforced or accidental degeneracy. The latter case provides greater flexibility in the design of topological switches, waveguides, and cloaking devices, because a large number of tuning parameters can be used to break the degeneracy and induce a topological phase. However, the design of a topological logic element--a switch that can be controlled by the output of a separate switch--remains elusive. Here we numerically demonstrate a topological logic gate for ultrasound by exploiting the large phase space of accidental degeneracies in a honeycomb lattice. We find that a degeneracy can be broken by six physical parameters, and we show how to tune these parameters to create a phononic switch that transitions between a topological waveguide and a trivial insulator by ultrasonic heating. Our design scheme is directly applicable to photonic crystals and may guide the design of future electronic topological transistors.

cond-mat.mes-hall↗